The present invention relates to the measurement of forces. In particular, the present invention relates to the measurement of mechanical forces, such as, for example, transversal forces (such as pressure) and rotational forces (such as torque). In more detail, the present invention relates to the measurement of forces by using optical transducers.
During recent years, much development work has been devoted to providing devices adapted to measure and/or detect mechanical forces in a very reliable manner. Among the devices and systems developed and proposed, systems and devices based on very sophisticated electronic assemblies became the most largely used devices and systems. This, in particular, was due to the fact that the development in the sector of integrated circuits and the corresponding reduction in size of circuits exploiting very complicated functions, allowed the provision of very small electronic transducers, adapted to be used for different purposes and under very difficult conditions. For instance, electronic transducers are known, the size of which is kept less than a few cubic millimeters. Moreover, the last developments in the field of the computing means, in particular, in the field of the software adapted for elaborating very big quantities of data in an always shorter time, allowed the data detected by the electronic transducers to be elaborated in an automatic and reliable manner. Finally, the decreasing costs of electronic systems allowed containment of the costs for producing electronic transducers, thus allowing such electronic transducers to be used for several purposes and applications.
However, in spite of the advantages offered by electronic transducers, electronic transducers are not free from drawbacks, especially when they are to be used for measuring mechanical forces, such as, for example, pressures and torques. The most relevant drawback affecting electronic transducers arises from the fact that electrical current is needed for operating the electronic transducers. In the case of a force acting on an electronic transducer, the electrical current flowing through the transducer is influenced by the force acting on it, so that the variations in the current flow may be detected and used for obtaining an indication of the intensity of the force acting on the transducer.
However, the electrical current flowing through the electronic transducers may also be influenced by the external environment, thus rendering electronic transducers less reliable for applications in critical environments, such as in structures exposed to electrostatic discharges during thunderstorms or in electromagnetically noisy industrial premises. Moreover, it may become difficult or risky to use electronic transducers in storage areas of highly flammable materials. Finally, some electronic transducers are also not suitable for certain biomedical applications because the risk of electrocution may arise. I
Some attempts have also been made in the last years for overcoming the drawbacks affecting electronic forces measurement systems. In particular, in the last years, some efforts have been devoted to the development of optical transducers. These optical transducers are based on the consideration that forces, in particular mechanical forces such as pressure or torque, may be measured and/or detected using evaluations of the effects on light transmitted through an optical path caused by a force acting, either directly or indirectly, on said optical path. In particular, the working principle of many of the known optical transducers exploits the variation in the photo-current detected at the output of an optical path with the optical signal attenuation caused by the variation of the optical link attenuation or by the signal interference that are controlled by the force under test. In fact, it has been observed that a relationship may be established between the photo-current detected at the output of an optical path with the mechanical stress acting on the mechanical path. In particular, some of these known optical transducers exploit the variation of polarization in optical fibers with induced micro-stresses caused by an external applied force. Unfortunately, however, these known optical transducers based on the variation of polarization are themselves not free from drawbacks which limit their use to only few applications. Moreover, the results of the measurements exploited with these known optical transducers are mostly not as reliable as desired. Finally, assembling and manufacturing these known optical transducers is quite cumbersome and, therefore, quite expensive since it requires stringent mechanical tolerances.
The drawbacks affecting these known optical transducers mostly arise since said optical transducers are based on the interference of two orthogonally polarized modes and require, therefore, special, highly birefringent fibers and very precise mechanical housings. In more detail, these known optical transducers are based on the detection of the variations in polarization of the light beam transmitted through a birefringent optical fiber; the application of a mechanical stress to the fiber produces a substantial number of interference fringes and a phase measuring system is used to determine the change in birefringence induced by the mechanical stress, and hence the value of the mechanical stress itself.
Accordingly, in view of the problems explained above, it would be desirable to provide a technology that may solve or reduce these problems, while maintaining all the positive characteristics of fiber based sensors. In particular, it would be desirable to provide transducers suitable to be used in structures exposed to electrostatic discharges and/or in noisy industrial premises, or even in storage areas of highly flammable materials. In the same way, it would be desirable to provide transducers for measuring and/or detecting forces suitable to be used for biomedical applications. Furthermore, it would be desirable to provide transducers characterized by low cost, light weight, reduced size and minimal invasiveness. It would also be desirable to provide transducers for the purpose of reliably measuring forces, which could be used in combination with low cost, simple and well known equipment. Finally, it would be desirable to provide transducers which allow those drawbacks to be overcome that affect the known optical transducers; in particular, it would be desirable to provide optical transducers based on standard single mode optical fibers (SMF).